Soil testing only the top 0 – 10cm of soil is not sufficient to predict if applying Potassium fertiliser will produce a yield response in a crop. The 0-10cm zone has traditionally been the most common type of soil sample to base fertiliser recommendations on.
The roots of crop plants commonly develop down to 1.8 m in the soil profile. If the roots reach soil with Potassium reserves anywhere in that zone, their grain yield will catch up to crops that had potassium fertiliser applied based on a shallow soil test. This happens even when an early vegetative response to potassium fertiliser is observed.
This means if you want soil information to base a Potassium fertiliser recommendation on, you have to test deeper in the soil profile. It’s not yet established what an adequate deeper soil sampling protocol for predicting a fertiliser response to Potassium.
Making greater use of other deep soil samples that inform crop management, such as deep N, is a useful way to approach monitoring Potassium for now.
Mark Conyers, NSW DPI, Jeff Kraak, Fertilizer Australia & Charlie Walker, Incitec Pivot.